For the Pancakes: Strain the potatoes and then put them in a kitchen towel along with the onions. Wring out the excess liquid. Put the potato and onion mixture in a large mixing bowl and combine the egg. Season with salt and pepper.

Heat a large cast iron pan over medium high heat. Once hot, add about 3 to 4 tablesoons of olive oil. Working in batches of 4 to 5 latkes (you don't want to overcrowd the pan), spoon 2 tablespoons potato mixture per latke into skillet, pressing into 3-inch rounds with a fork. Reduce heat to moderate and cook until undersides are browned, about 5 minutes. Carefully turn latkes over and cook until the other sides are browned, about 5 minutes more. Transfer to paper towels to drain and season with salt. Add more oil to skillet as needed. And keep working with the remaining potato mixture. Keep latkes warm by covering the plate with foil.

For the Slaw: In a medium size mixing bowl, add the vinegar, mustard and honey. Drizzle in the olive oil and whisk to emulsify. Toss in the apples and the capers and mix to combine. Season with salt and pepper.

Top each latke with a bit of the apple slaw. Grate horseradish over each latke if desired.